**2. Construction of artificial proteins**

The scheme of the screening process of an artificial protein or enzyme using directed evolution technology is shown in **Figure 1**. The screening process in a single cycle is composed of three steps: (1) construction of DNA (or RNA) variants, (2) construction of display library composed of both DNA (or RNA) variants and protein molecules, and (3) screening of target DNA variants. Directed evolution is completed by several repeats of these steps to obtain an optimal enzyme (or protein). Each step is discussed in the following sections.
